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<h3><a name="SECTION0016328000000000000000"></a><a name="optparse-how-optparse-handles-errors"></a>
<br>
14.3.2.8 How <tt class="module">optparse</tt> handles errors
</h3>

<p>
There are two broad classes of errors that <tt class="module">optparse</tt> has to worry about:
programmer errors and user errors.  Programmer errors are usually
erroneous calls to <code>parser.add_option()</code>, e.g. invalid option strings,
unknown option attributes, missing option attributes, etc.  These are
dealt with in the usual way: raise an exception (either
<code>optparse.OptionError</code> or <code>TypeError</code>) and let the program crash.

<p>
Handling user errors is much more important, since they are guaranteed
to happen no matter how stable your code is.  <tt class="module">optparse</tt> can automatically
detect some user errors, such as bad option arguments (passing <code>"-n
4x"</code> where <b class="programopt">-n</b> takes an integer argument), missing arguments
(<code>"-n"</code> at the end of the command line, where <b class="programopt">-n</b> takes an argument
of any type).  Also, you can call <code>parser.error()</code> to signal an
application-defined error condition:
<div class="verbatim"><pre>
(options, args) = parser.parse_args()
[...]
if options.a and options.b:
    parser.error("options -a and -b are mutually exclusive")
</pre></div>

<p>
In either case, <tt class="module">optparse</tt> handles the error the same way: it prints the
program's usage message and an error message to standard error and
exits with error status 2.

<p>
Consider the first example above, where the user passes <code>"4x"</code> to an
option that takes an integer:
<div class="verbatim"><pre>
$ /usr/bin/foo -n 4x
usage: foo [options]

foo: error: option -n: invalid integer value: '4x'
</pre></div>

<p>
Or, where the user fails to pass a value at all:
<div class="verbatim"><pre>
$ /usr/bin/foo -n
usage: foo [options]

foo: error: -n option requires an argument
</pre></div>

<p>
<tt class="module">optparse</tt>-generated error messages take care always to mention the option
involved in the error; be sure to do the same when calling
<code>parser.error()</code> from your application code.

<p>
If <tt class="module">optparse</tt>'s default error-handling behaviour does not suite your needs,
you'll need to subclass OptionParser and override <code>exit()</code> and/or
<tt class="method">error()</tt>.

<p>

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